Furthermore, biopolymer-based superabsorbent polymers (biopolymer-SAPs), which are 3D cross-linked hydrophilic polymers that can absorb and hold a large amount of water (Xie et al., 2013; Zohuriaan-Mehr & Kabiri, 2008), are potential materials for regulating the release behaviors of fertilizers because of their electric charges and interconnected open channels in the 3D gel network. It is worth noting that a multi-layer slow-release fertilizer with steady nutrient release behaviors could be prepared by using the fertilizer core with PVA as the first coating layer, followed by cross-linked chitosan and poly-(acrylic-co-acrylamide) as the second and third layers, respectively.